Spin manipulation is used in spintronics, or spin electronics, which term refers to the technology that offers opportunities for a new generation of devices that exploit electron spin information, rather than charge. Spins, being the attribute of mobile electrons, can carry the information along the conductive line. In turn, the information can be read afterwards.
Spin orientation of conduction electrons survives for a relatively long time (e.g., nanoseconds). This feature makes spintronic devices particularly attractive for memory storage, magnetic sensor applications, and eventually for quantum information processing and quantum computation where electron spin would represent a bit (usually referred to as a “qubit”) of information [1-4].
Spintronic devices are already in use in industry as a read head and a memory-storage cell. For example, the read head is the giant-magnetoresistive (GMR) sandwich structure that includes alternating ferromagnetic and nonmagnetic metal layers [4]. Depending on the relative orientation of the magnetization in the magnetic layers, the device resistance changes from small (parallel magnetizations) to large (antiparallel magnetizations). This change in resistance is used to sense changes in magnetic fields. Another example of the spintronic application is related to tunneling magneto resistance (CMR) and magnetic tunnel junction (MTJ) devices where the tunneling current depends on spin orientations of the electrodes. A typical MFJ device includes two magnetic layers separated by an insulating metal-oxide layer. Electrons can “tunnel” through from one layer to the other, only when magnetizations of the layers point in the same direction, otherwise, the resistance is high.
One of the approaches of the current efforts in designing and manufacturing spintronic devices involves developing new materials and structures that exhibit a high level of spin polarization of spin carriers. Most efforts in this approach are currently concentrated on ferromagnetic semiconductors and combinations of semiconductors and ferromagnetic metals [4-6]. The central idea of the prior art techniques is to polarize electrons using ferromagnetic semiconductors, or one of the novel composite magnetic structures, with the subsequent injection of the polarized electrons into a semiconductor device for further applications [1-6].
It is possible, in principle, to create a spin-polarized current in a non-ferromagnetic material by creating a spin-polarized electron current in a ferromagnetic material and causing the current to flow from the ferromagnetic material into an adjacent non-ferromagnetic material by way of an interface. Spin-polarized electron transport has been achieved in this manner for example, from ferromagnetic metals into normal metals [7] and from magnetic semiconductors into non-magnetic semiconductors (see [8], [9]).
The difficulties of the technology of the injection of the spin polarized electrons into a semiconductor system have a general character. The spin injection from a ferromagnetic metal into a semiconductor structure is strongly suppressed [10]. One of the suggestions of how to overcome these difficulties is described in [5]. A method and apparatus which create a spin filter at an interface between a semiconductor and a ferromagnetic material has been suggested. The spin filter can be used to provide a current of spin-polarized charge carriers in the semiconductor. The spintronic device comprises a crystalline first semiconductor; and a crystalline ferromagnetic material in atomic registration with the first semiconductor at an interface. The semiconductor and ferromagnetic material were chosen so that transmission of current carriers in a first spin state from the ferromagnetic material into the first semiconductor is quantum mechanically forbidden, while the transmission of current carriers in a second spin state from the ferromagnetic material into the first semiconductor is quantum mechanically permitted.